1. Field of the Invention
This invention relates generally to global navigation satellite systems (GNSS) and more particularly to short baseline receivers.
2. Background Information
Short baseline real time kinematic (RTK) systems typically operate with a base GNSS receiver and a rover GNSS receiver that are separated by a small number of kilometers, for example, less than 10 kilometers. The base receiver, which is in a known position, makes carrier phase measurements using GNSS signals transmitted from GNSS satellites in view and calculates pseudoranges from the respective satellites. The base receiver then determines differences between the pseudoranges calculated using the satellite signals and the ranges based on the known position of the receiver and the known locations of the satellites, to determine range correction information. The base receiver operating in a known manner broadcasts the RTK information, that is, the range correction information, pseudoranges, carrier phase measurements and various other information.
The rover receiver utilizes the broadcast pseudoranges, carrier phase measurements and other information to solve for integer carrier cycle ambiguities using well known, processing intensive, operations. The rover receiver utilizes the range correction information to correct for pseudorange errors related to changes in satellite orbits, atmospheric conditions, and so forth, that affect both the base receiver and the rover receiver in the same manner due to the short baseline between the receivers, all in a known manner.
Certain short baseline systems utilize fixed baselines that may, for example, employ two antennas situated at the rover receiver to determine the orientation or azimuth of the rover receiver. The antennas may be fixed to a vehicle, such as an automobile or a ship, and may, for example, be spaced apart by as little as 1 to ½ meter. The multiple antennas provide information that is utilized in well known short baseline RTK processing operations to resolve carrier cycle ambiguities for the respective antennas. Once the carrier cycle ambiguities are resolved, the system can then determine the azimuth or orientation of the vehicle based on the differences in the carrier phases at the two antennas.
The fixed baseline systems suffer from the adverse effects of multipath and biases introduced by receiver operations. The signals received at each antenna may be affected in the same or different manners by the multipath signals. Further, the multipath signals may also differ across the GNSS satellites and at different times, depending on the locations of the respective GNSS satellites. The line biases may differ also, depending on the ambient environment, and so forth.
A co-pending application Ser. No. 12/579,460 entitled Ultra-Short Baseline GNSS Receiver, that is filed on even date herewith and assigned to a common Assignee and which is hereby incorporated herein in its entirety by reference, describes a system that utilizes two antennas that are spaced apart by less than one wavelength of the GNSS satellite carrier signal of interest. These antennas are referred to herein as being on an “ultra-short” fixed baseline. Such a system has the advantage that the integer carrier cycle ambiguities are not a problem between measurements made by the two antennas, and orientation or azimuth can be determined without the use of the processing intensive short base line RTK operations. However, the close proximity of the antennas may adversely affect the received signals due, for example, to cross-talk between the antennas.